A known power conversion device with a three level (PWM) converter has two DC voltage detectors that are provided for two filter capacitors, respectively, connected with a DC side of the three level converter for detecting the individual voltages of the capacitors, and the three level converter is PWM controlled based on the voltages of the filter capacitors detected by the two DC voltage detectors, respectively (see, for example, a first patent document).
However, when a voltage across opposite ends of each filter capacitor becomes higher than an ordinary voltage owing to a rapid variation of a load or the like, an abnormal voltage will be applied to a semiconductor device of the three level converter.
In addition, in order to prevent a detrimental effect as referred to above, there is a method of connecting an overvoltage suppression circuit comprising an overvoltage suppression resistor and a thyristor in parallel with the two filter capacitors. In operation, when a voltage across opposite ends of one filter capacitor becomes higher than a predetermined value, it is detected by an overvoltage detector, and the thyristor is fired to short-circuit that filter capacitor through the overvoltage suppression resistor, so that the energy accumulated in that filter capacitor is released whereby an AC switch arranged on a power supply side of the three level converter is opened after a predetermined time, thus cutting off a follow or dynamic current (see, for example, a second patent document).
In this method, however, when the AC switch does not perform normal opening operation due to a failure or the like, a current detector for detecting a current flowing through the overvoltage suppression resistor is required so as not to invite a heated burnout of the overvoltage suppression resistor, thus resulting in that the device becomes excessively large.
Accordingly, in order to solve the problems as referred to above, there is a method of detecting false firing of a thyristor in an electric car control apparatus with a three level (PWM) inverter, in which a first voltage sensor is connected in parallel to the thyristor to detect an intermediate direct current full voltage. Here, a second voltage sensor is connected in parallel to a filter capacitor, which is connected to a negative terminal side of a terminal of the three level inverter, for detecting a downstream half voltage in the form of a terminal to terminal voltage of the filter capacitor, whereby the pulse width of the three level inverter is controlled by using the voltage values detected by the first and second voltage sensors so as to make the value of an upstream half voltage and the value of the downstream half voltage of the intermediate DC voltage equal to each other (see, for example, a third patent document).
In this case, the upstream half voltage, which is a terminal to terminal voltage of the filter capacitor connected to a positive terminal side of the three level inverter, is a value which is obtained by subtracting the downstream voltage detected by the second voltage sensor connected in parallel to the filter capacitor connected to a negative terminal side of the terminal of the three level inverter from the full voltage detected by the first voltage sensor connected in parallel to the thyristor. However, in case where the resolution of the first voltage sensor is lower than the resolution of the second voltage sensor, the upstream half voltage calculated from the voltage detected from these two voltage sensors is lower in accuracy than the downstream half voltage directly detected, and as a result, the accuracy of the control also becomes low. Accordingly, in order to perform control according to this voltage detection system, it is necessary to match the resolutions of the voltage sensors to the lower one. Here, note that a similar discussion can be made for the three level converter.
[First Patent Document] Japanese patent application laid-open No. H 11-113263
[Second Patent Document] Japanese patent application laid-open No. H 7-154974
[Third Patent Document] Japanese patent application laid-open No. H 8-33102